Gas sensor

ABSTRACT

A sensor element includes: a main pump cell constituted by an inner pump electrode facing the first inner space into which a measurement gas is introduced, an external pump electrode provided on an element surface, and a solid electrolyte located therebetween; a measurement electrode facing a second inner space communicating with the first inner space and functioning as a reduction catalyst for NOx; and a measurement pump cell constituted by the measurement electrode, the external pump electrode, and a solid electrolyte located therebetween. A diffusion resistance from the gas inlet to the inner pump electrode ranges from 200-1000 cm −1 , a resistance of the main pump cell is equal to or smaller than 150Ω, a distance between the electrodes ranges from 0.1-0.6 mm, and the inner pump electrode which is a cermet made of an Au—Pt alloy and ZrO 2  has an area ranging from 5-20 mm 2 .

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority from Japanese application JP2018-049372, filed on Mar. 16, 2018, the contents of which is herebyincorporated by reference into this application.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a gas sensor obtaining a concentrationof nitrogen oxide (NOx), and particularly to ensuring of accuracy in ahigh NOx concentration region.

Description of the Background Art

Already known is a limiting current type gas sensor (NOx sensor) using asensor element which mainly has an oxygen ion conductive solidelectrolyte as a constituent (for example, see Japanese Patent No.3050781). In order to obtain the NOx concentration in such a gas sensor,a measurement gas is firstly introduced into a space provided inside thesensor element (an inner space) under a predetermined diffusionresistance, and oxygen in the measurement gas is pumped out in anelectrochemical pump cell provided in two stages such as a main pumpcell and an auxiliary pump cell (first and second electrochemical pumpcells in Japanese Patent No. 3050781) to sufficiently lower the oxygenconcentration in the measurement gas previously. Subsequently, NOx inthe measurement gas is reduced or resolved in a measurement electrodefunctioning as a reduction catalyst (a third inner pump electrode inJapanese Patent No. 3050781), and oxygen generated by the reduction orthe resolution is pumped out in an electrochemical pump cell includingthe measurement electrode other than the pump cell described above,called a measurement pump cell, for example (a third electrochemicalpump cell in Japanese Patent No. 3050781). The concentration of NOx isobtained by using a fact that current flowing in the measurement pumpcell (NOx current) has a certain functional relationship with theconcentration of NOx.

Also already known is an embodiment that in the gas sensor (NOx sensor),Pt to which Au is added (Au—Pt alloy) is used as a metal component of aninner pump electrode provided in an inner space to constitute a mainpump cell, for purpose of suppressing the resolution of NOx when themain pump cell pumps oxygen out of the inner space and increasing adetection accuracy of NOx (for example, see Japanese Patent ApplicationLaid-Open No. 2014-190940 and Japanese Patent Application Laid-Open No.2014-209128).

In the gas sensor described above, the concentration of NOx is obtainedbased on an amount of oxygen generated by a reduction of NOx in themeasurement gas reaching the measurement electrode due to catalyticaction of the measurement electrode. At this time, oxygen in themeasurement gas is pumped out by the electrochemical pump cell until themeasurement gas reaches the measurement electrode, and this pumping-outof oxygen is performed so that the oxygen partial pressure (oxygenconcentration) of the measurement gas is lowered enough to the extentnot to resolve NOx. The reason is that if NOx is resolved before themeasurement gas reaches the measurement electrode, the amount of NOxreaching the measurement electrode decreases, thus the concentrationcannot be obtained accurately.

However, when the oxygen concentration of the measurement gas introducedinto the inner space is high, NOx may be resolved at the time of pumpingout oxygen. Obtained after an earnest review by the inventor of thepresent invention are findings that, due to a tendency where the oxygenconcentration of the measurement gas in the inner space is higher in aportion closer to an upstream side (in a side closer to a gas inlet ofthe sensor element), a high pump voltage tends to be locally applied ina portion closer to an upstream side of the inner pump electrode inorder to carry out to pump out oxygen from the measurement gas whoseoxygen concentration is high, and NOx is resolved in such a portion.

SUMMARY

The present invention relates to a gas sensor obtaining a concentrationof nitrogen oxide (NOx), and particularly to ensuring of accuracy in ahigh NOx concentration region.

According to the present invention, a limiting current type gas sensorcapable of specifying a concentration of NOx in a measurement gasincludes: a sensor element formed of an oxygen ion conductive solidelectrolyte, wherein the sensor element includes: a gas inlet into whicha measurement gas is introduced from an outer space; a first inner spacecommunicated with the gas inlet under a predetermined diffusionresistance; a second inner space communicated with the first inner spaceunder a predetermined diffusion resistance; a main pump cell which is anelectrochemical pump cell constituted by an inner pump electrodeprovided to face the first inner space, an external pump electrodeprovided on a surface of the sensor element, and the solid electrolytelocated between the inner pump electrode and the external pumpelectrode; a measurement electrode provided to face the second innerspace and covered with a porous protection film providing apredetermined diffusion resistance, said measurement electrodefunctioning as a reduction catalyst for NOx; an atmospheric airintroduction layer into which atmospheric air is introduced from outsideof the sensor element as a reference gas; a reference electrode coveredwith the atmospheric air introduction layer; and a measurement pump cellwhich is an electrochemical pump cell constituted by the measurementelectrode, the external pump electrode, and the solid electrolytelocated between the measurement electrode and the external pumpelectrode; and a concentration specifying element specifying aconcentration of the NOx based on a magnitude of an NOx current flowingbetween the measurement electrode and the external pump electrode in themeasurement pump cell, wherein the main pump cell is configured anddisposed to pump out oxygen in the first inner space when apredetermined main pump voltage is applied between the inner pumpelectrode and the external pump electrode, and pumps out oxygen in themeasurement gas introduced into the first inner space to lower oxygenpartial pressure of the measurement gas in the first inner space, themeasurement pump cell is configured and disposed to pump out oxygen nearthe measurement electrode when a predetermined pump voltage is appliedbetween the inner pump electrode and the external pump electrode, andpumps out oxygen generated by a reduction of NOx in the measurement gasreaching near the measurement electrode in the measurement electrode, adiffusion resistance from the gas inlet to the inner pump electrode isequal to or larger than 200 cm⁻¹ and equal to or smaller than 1000 cm⁻¹,an electrical resistance of the main pump cell is equal to or smallerthan 150Ω, a shortest distance from the inner pump electrode to theexternal pump electrode is equal to or larger than 0.1 mm and equal toor smaller than 0.6 mm, and the inner pump electrode is a cermetelectrode formed of an Au—Pt alloy and ZrO₂ having an area equal to orlarger than 5 mm² and equal to or smaller than 20 mm².

According to the present invention, even when the oxygen concentrationof the measurement gas is high, the resolution of NOx in the first innerspace is preferably suppressed, thus the concentration of NOx in themeasurement gas can be accurately obtained.

Accordingly, an object of the present invention is to provide a gassensor capable of measuring NOx accurately even when the oxygenconcentration in the measurement gas is high.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing schematically showing an example of a configurationof a gas sensor 100.

FIG. 2 is a drawing showing an influence of a magnitude of a main pumpvoltage Vp0 on a relationship between an oxygen concentration of ameasurement gas and an NOx current Ip2.

FIG. 3 is a drawing showing a flow of processing in manufacturing asensor element 101.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

<Schematic Configuration of Gas Sensor>

Described first is a schematic configuration of a gas sensor 100including a sensor element 101 according to the present embodiment. Inthe present embodiment, the gas sensor 100 is a limiting current typeNOx sensor which detects NOx using the sensor element 101 to measure aconcentration of NOx.

FIG. 1 is a drawing schematically showing an example of a configurationof the gas sensor 100 including a vertical sectional view of the sensorelement 101 along a longitudinal direction.

The sensor element 101 is a flat plate like (elongated plate like)element having a structure made up of six solid electrolyte layers of afirst substrate layer 1, a second substrate layer 2, a third substratelayer 3, a first solid electrolyte layer 4, a spacer layer 5, and asecond solid electrolyte layer 6, each of which is formed of zirconia(ZrO₂) which is an oxygen ion conductive solid electrolyte (for example,yttrium stabilized zirconia (YSZ)), laminated from a lower side in thisorder when seeing a drawing sheet of FIG. 1. The solid electrolyteforming these six layers is dense and airtight. In the subsequentdescription, a surface on an upper side of each of these six layers inFIG. 1 is simply referred to as an upper surface, and a surface on alower side thereof is simply referred to as a lower surface in somecases. A whole part made of the solid electrolyte in the sensor element101 is collectively referred to as a base part.

The sensor element 101 is manufactured by performing a predeterminedprocessing and printing a circuit pattern on a ceramic green sheetcorresponding to each layer, then laminating the green sheets, andfurther firing to integrate them with each other, for example.

A gas inlet 10, a first diffusion limiting part 11, a buffer space 12, asecond diffusion limiting part 13, a first inner space 20, a thirddiffusion limiting part 30, and a second inner space 40 are adjacentlyformed to be communicated with each other in this order between a lowersurface of the second solid electrolyte layer 6 and an upper surface ofthe first solid electrolyte layer 4 in one end of the sensor element101.

The gas inlet 10, the buffer space 12, the first inner space 20, and thesecond inner space 40 are spaces in the sensor element 101 that look asif they were provided by hollowing out the spacer layer 5, an upper partthereof defined by the lower surface of the second solid electrolytelayer 6, a lower part thereof defined by the upper surface of the firstsolid electrolyte layer 4, and a side part thereof defined by the sidesurface of the spacer layer 5.

Each of the first diffusion limiting part 11, the second diffusionlimiting part 13, and the third diffusion limiting part 30 is providedas two horizontally long slits (with an opening having a longitudinaldirection perpendicular to the drawing sheet of FIG. 1). A region fromthe gas inlet 10 to the second inner space 40 is also referred to as agas distribution part.

A reference gas introduction space 43 is provided in a position fartheraway from an end side in relation to the gas introduction part betweenthe upper surface of the third substrate layer 3 and the lower surfaceof the spacer layer 5, a side part thereof defined by a side surface ofthe first solid electrolyte layer 4. Atmospheric air, for example, isintroduced into the reference gas introduction space 43 as a referencegas in measuring the NOx concentration.

An atmospheric air introduction layer 48 is a layer formed of porousalumina, and the reference gas is introduced into the atmospheric airintroduction layer 48 through the reference gas introduction space 43.The atmospheric air introduction layer 48 is formed to cover a referenceelectrode 42.

The reference electrode 42 is an electrode having a configuration ofbeing sandwiched between the upper surface of the third substrate layer3 and the first solid electrolyte layer 4, and the atmospheric airintroduction layer 48 leading to the reference gas introduction space 43is provided around the reference electrode 42 as described above. Anoxygen concentration (oxygen partial pressure) in the first inner space20 and the second inner space 40 can be measured using the referenceelectrode 42 as described hereinafter.

The gas inlet 10 is a portion having an opening to an outer space in thegas introduction part, and the measurement gas is taken into the sensorelement 101 from the outer space through the gas inlet 10.

The first diffusion limiting part 11 is a portion for providing themeasurement gas taken from the gas inlet 10 of the predetermineddiffusion resistance.

The buffer space 12 is a space provided for leading the measurement gas,which is introduced from the first diffusion limiting part 11, to thesecond diffusion limiting part 13.

The second diffusion limiting part 13 is a portion for providing themeasurement gas introduced from the buffer space 12 to the first innerspace 20 of the predetermined diffusion resistance.

In the introduction of the measurement gas from outside the sensorelement 101 into the first inner space 20, the measurement gas rapidlytaken into the sensor element 101 from the gas inlet 10 in accordancewith a pressure variation of the measurement gas in the outer space (apulsation of an exhaust gas pressure in a case where the measurement gasis an exhaust gas of a vehicle) is not directly introduced into thefirst inner space 20, but is introduced into the first inner space 20after a concentration variation of the measurement gas is canceledthrough the first diffusion limiting part 11, the buffer space 12, andthe second diffusion limiting part 13. Thus, the concentration variationof the measurement gas introduced into the first inner space 20 issubstantially negligible.

The first inner space 20 is provided as a space for adjusting the oxygenpartial pressure in the measurement gas introduced through the seconddiffusion limiting part 13. The oxygen partial pressure is adjusted byan operation of a main pump cell 21.

The main pump cell 21 is an electrochemical pump cell constituted by theinner pump electrode 22 having a ceiling electrode part 22 a provided onalmost the entire lower surface of the second solid electrolyte layer 6facing the first inner space 20, an external pump electrode 23 providedto be exposed to the outer space in a region corresponding to theceiling electrode part 22 a on the upper surface of the second solidelectrolyte layer 6 (one main surface of the sensor element 101), andthe second solid electrolyte layer 6 sandwiched between the electrodes22 and 23.

The inner pump electrode 22 is formed on the solid electrolyte layers onan upper side and a lower side defining the first inner space 20 (thesecond solid electrolyte layer 6 and the first solid electrolyte layer4). Specifically, the ceiling electrode part 22 a is formed on the lowersurface of the second solid electrolyte layer 6 providing a ceilingsurface of the first inner space 20, and a bottom electrode part 22 b isformed on the upper surface of the first solid electrolyte layer 4providing a bottom surface of the first inner space 20. The ceilingelectrode part 22 a and the bottom electrode part 22 b are connected toeach other in a conduction part provided on a sidewall surface (an innersurface) of the spacer layer 5 constituting both sidewall parts of thefirst inner space 20 (the illustration is omitted).

The ceiling electrode part 22 a and the bottom electrode part 22 b areprovided to have a rectangular shape in a plan view. However, aconfiguration of providing only the ceiling electrode part 22 a or onlythe bottom electrode part 22 b may also be applicable.

Each of the inner pump electrode 22 and the external pump electrode 23are formed as a porous cermet electrode. Particularly, the inner pumpelectrode 22 contacting the measurement gas is formed using a materialwhose reducing ability on an NOx component in the measurement gas isweakened. For example, the inner pump electrode 22 is formed to have aporosity ranging from 5% to 40% and a thickness ranging from 5 μm to 20μm as a cermet electrode made of an Au—Pt alloy containing Ausubstantially equal to or larger than 0.6 wt % and equal to or smallerthan 1.4 wt % and ZrO₂. A weight ratio of the Au—Pt alloy to ZrO₂ may beapproximately Pt:ZrO₂=7.0:3.0 to 5.0:5.0.

In the meanwhile, the external pump electrode 23 is formed to have arectangular shape in a plan view as a cermet electrode made of Pt or aPt alloy and ZrO₂, for example.

In the main pump cell 21, a desired pump voltage Vp0 is applied betweenthe inner pump electrode 22 and the external pump electrode 23 by avariable source 24, and a pump current Ip0 is flowed between the innerpump electrode 22 and the external pump electrode 23 in a positivedirection or a negative direction, thus oxygen in the first inner space20 can be pumped out to the outer space or oxygen in the outer space canbe pumped into the first inner space 20. The pump voltage Vp0 appliedbetween the inner pump electrode 22 and the external pump electrode 23in the main pump cell 21 is also referred to as the main pump voltageVp0.

The inner pump electrode 22, the second solid electrolyte layer 6, thespacer layer 5, the first solid electrolyte layer 4, the third substratelayer 3, and the reference electrode 42 constitute an electrochemicalsensor cell, that is to say, a main-pump-control oxygen-partial-pressuredetection sensor cell 80 to detect the oxygen concentration (oxygenpartial pressure) in the atmosphere in the first inner space 20.

The oxygen concentration (oxygen partial pressure) in the first innerspace 20 can be figured out by measuring an electromotive force V0 inthe main-pump-control oxygen-partial-pressure detection sensor cell 80.

Furthermore, a feedback control is performed on the main pump voltageVp0 so that the electromotive force V0 is set to constant, thus the pumpcurrent Ip0 is controlled. Accordingly, the oxygen concentration in thefirst inner space 20 is maintained to have a predetermined constantvalue.

The third diffusion limiting part 30 is a portion of providing themeasurement gas, whose oxygen concentration (oxygen partial pressure) iscontrolled by an operation of the main pump cell 21 in the first innerspace 20, of a predetermined diffusion resistance, and guiding themeasurement gas to the second inner space 40.

The second inner space 40 is provided as a space for performingprocessing according to the measurement of nitrogen oxide (NOx) in themeasurement gas introduced through the third diffusion limiting part 30.The NOx concentration is measured mainly in the second inner space 40where the oxygen concentration is adjusted by an auxiliary pump cell 50,by an operation of a measurement pump cell 41.

The oxygen concentration (oxygen partial pressure) is previouslyadjusted in the first inner space 20, and subsequently, in the secondinner space 40, the adjustment of the oxygen partial pressure is furtherperformed by the auxiliary pump cell 50 on the measurement gasintroduced through the third diffusion limiting part 30. Accordingly,the oxygen concentration in the second inner space 40 can be accuratelymaintained constant, thus the gas sensor 100 enables the highly accurateNOx concentration measurement.

The auxiliary pump cell 50 is an auxiliary electrochemical pump cellconstituted by an auxiliary pump electrode 51 having a ceiling electrodepart 51 a provided on almost the entire lower surface of the secondsolid electrolyte layer 6 facing the second inner space 40, the externalpump electrode 23 (not limited to the external pump electrode 23 but anappropriate electrode outside the sensor element 101 is alsoapplicable), and the second solid electrolyte layer 6.

The auxiliary pump electrode 51 is disposed in the second inner space 40similarly to the inner pump electrode 22 provided in the first innerspace 20 described above. In other words, the ceiling electrode part 51a is formed on the second solid electrolyte layer 6 providing a ceilingsurface of the second inner space 40, and a bottom electrode part 51 bis formed on the first solid electrolyte layer 4 providing a bottomsurface of the second inner space 40. Each of the ceiling electrode part51 a and the bottom electrode part 51 b has a rectangular shape in aplan view and is connected to each other in a conduction part providedon a sidewall surface (an inner surface) of the spacer layer 5constituting both sidewall parts of the second inner space 40 (theillustration is omitted).

In the manner similar to the inner pump electrode 22, the auxiliary pumpelectrode 51 is also formed using a material whose reducing ability onan NOx component in the measurement gas is weakened.

In the auxiliary pump cell 50, a desired pump voltage Vp1 is appliedbetween the auxiliary pump electrode 51 and the external pump electrode23, thus oxygen in the atmosphere in the second inner space 40 can bepumped out to the outer space or oxygen can be pumped from the outerspace into the second inner space 40.

The auxiliary pump electrode 51, the reference electrode 42, the secondsolid electrolyte layer 6, the spacer layer 5, the first solidelectrolyte layer 4, and the third substrate layer 3 constitute anelectrochemical sensor cell, that is to say, an auxiliary-pump-controloxygen-partial-pressure detection sensor cell 81 to control the oxygenpartial pressure in the atmosphere in the second inner space 40.

The auxiliary pump cell 50 performs pumping with a variable source 52 onwhich a voltage control is performed based on an electromotive force V1detected in the auxiliary-pump-control oxygen-partial-pressure detectionsensor cell 81. Accordingly, the oxygen partial pressure in theatmosphere in the second inner space 40 is controlled so that it is lowenough not to substantially influence the measurement of NOx.

In accordance with this, a pump current Ip1 thereof is used forcontrolling the electromotive force of the main-pump-controloxygen-partial-pressure detection sensor cell 80. Specifically, the pumpcurrent Ip1 is input, as a control signal, into the main-pump-controloxygen-partial-pressure detection sensor cell 80, and, through controlof the electromotive force V0 thereof, the oxygen partial pressure inthe measurement gas introduced through the third diffusion limiting part30 into the second inner space 40 is controlled to have a gradient thatis always constant. In using the gas sensor 100 as an NOx sensor, theoxygen concentration in the second inner space 40 is maintained to havea constant value of approximately 0.001 ppm by the functions of the mainpump cell 21 and the auxiliary pump cell 50.

The measurement pump cell 41 measures the NOx concentration in themeasurement gas in the second inner space 40. The measurement pump cell41 is an electrochemical pump cell constituted by a measurementelectrode 44 provided on the upper surface of the first solidelectrolyte layer 4 facing the second inner space 40 in a positionseparated from the third diffusion limiting part 30, the external pumpelectrode 23, the second solid electrolyte layer 6, the spacer layer 5,and the first solid electrolyte layer 4.

The measurement electrode 44 is a porous cermet electrode. For example,the measurement electrode 44 is formed as a cermet electrode made of Ptor an alloy of Pt and ZrO₂. The measurement electrode 44 also functionsas an NOx reduction catalyst for reducing NOx in the atmosphere in thesecond inner space 40. Furthermore, the measurement electrode 44 iscovered with a fourth diffusion limiting part 45.

The fourth diffusion limiting part 45 is a film formed of a porousmaterial mainly containing alumina (Al₂O₃). The fourth diffusionlimiting part 45 has a function of limiting an amount of NOx flowinginto the measurement electrode 44, and also functions as a protectionfilm of the measurement electrode 44.

The measurement pump cell 41 can pump out oxygen generated by theresolution of NOx in the atmosphere around the measurement electrode 44and detect a generation amount of oxygen as a pump current Ip2.

The second solid electrolyte layer 6, the spacer layer 5, the firstsolid electrolyte layer 4, the third substrate layer 3, the measurementelectrode 44, and the reference electrode 42 constitute anelectrochemical sensor cell, that is to say, a measurement-pump-controloxygen-partial-pressure detection sensor cell 82 to detect the oxygenpartial pressure around the measurement electrode 44. A variable source46 is controlled based on an electromotive force V2 detected in themeasurement pump control oxygen partial pressure detection sensor cell82.

The measurement gas introduced into the second inner space 40 reachesthe measurement electrode 44 through the fourth diffusion limiting part45 under a condition where the oxygen partial pressure is controlled.NOx in the measurement gas around the measurement electrode 44 isreduced (2NO→N₂+O₂), and oxygen is generated. The generated oxygen ispumped by the measurement pump cell 41. At this time, a voltage Vp2 ofthe variable source 46 is controlled so that a control voltage V2detected in the measurement pump control oxygen partial pressuredetection sensor cell 82 is set to constant. Since the amount of oxygengenerated around the measurement electrode 44 is proportional to the NOxconcentration in the measurement gas, the NOx concentration in themeasurement gas is calculated using the pump current Ip2 in themeasurement pump cell 41. The pump current Ip2 is also referred to asthe NOx current Ip2 hereinafter.

If the measurement electrode 44, the first solid electrolyte layer 4,the third substrate layer 3, and the reference electrode 42 are combinedto constitute an oxygen-partial-pressure detection means as anelectrochemical sensor cell, an electromotive force in accordance with adifference of an amount of oxygen generated by the reduction of the NOxcomponent in the atmosphere around the measurement electrode 44 and anamount of oxygen contained in a reference atmosphere can be detected,and accordingly, a concentration of the NOx component in the measurementgas can be also obtained.

The second solid electrolyte layer 6, the spacer layer 5, the firstsolid electrolyte layer 4, the third substrate layer 3, the externalpump electrode 23, and the reference electrode 42 constitute anelectrochemical sensor cell 83, and the oxygen partial pressure in themeasurement gas outside the sensor can be detected by an electromotiveforce Vref obtained by the sensor cell 83.

The sensor element 101 further includes a heater part 70 having afunction of adjusting a temperature for heating sensor element 101 andkeeping the temperature, in order to increase oxygen ion conductivity ofthe solid electrolyte constituting the base part.

The heater part 70 mainly includes a heater electrode 71, a heaterelement 72, a heater lead 72 a, a through hole 73, and a heaterinsulating layer 74. The heater part 70 is embedded in the base part ofthe sensor element 101 except for the heater electrode 71.

The heater electrode 71 is an electrode formed to contact the lowersurface of the first substrate layer 1 (the other main surface of thesensor element 101).

The heater element 72 is a resistance heating element provided betweenthe second substrate layer 2 and the third substrate layer 3. The heaterelement 72 generates the heat by supplying power from the outside of thesensor element 101 via the heater electrode 71, the through hole 73, andthe heater lead 72 a which function as an energizing path. The heaterelement 72 is formed of Pt or mainly of Pt. The heater element 72 isembedded in a predetermined region in the sensor element 101 on a sideincluding the gas introduction part so as to face the gas introductionpart in a thickness direction of the element. The heater element 72 isprovided to have a thickness of approximately 10 μm to 20 μm.

In the sensor element 101, the current is flowed into the heater element72 via the heater electrode 71, thereby making the heater element 72generate the heat, thus each part of the sensor element 101 can beheated to a predetermined temperature and kept to have the temperature.Specifically, the sensor element 101 is heated so that the temperatureof the solid electrolyte and the electrode near the gas introductionpart increases to approximately 700° C. to 900° C. The heatingprocessing increases the oxygen ion conductivity of the solidelectrolyte constituting the base part in the sensor element 101. Theheating temperature at the time of heating by the heater element 72 in acase of using the gas sensor 100 (in a case of driving the sensorelement 101) is referred to as a sensor element driving temperature.

The gas sensor 100 further includes a controller 110 controlling theoperation of each part and specifying the NOx concentration based on theNOx current Ip2.

In the gas sensor 100 having such a configuration, oxygen contained inthe measurement gas is pumped out through the operation of the main pumpcell 21 and further of the auxiliary pump cell 50, and the measurementgas whose oxygen partial pressure is lowered enough not to substantiallyinfluence the measurement of NOx (for example, 0.0001 ppm to 1 ppm)reaches the measurement electrode 44. In the measurement electrode 44,NOx in the measurement gas which has reached the measurement electrode44 is reduced, and oxygen is generated. The generated oxygen is pumpedout by the measurement pump cell 41. The NOx current Ip2 flowing at thetime of pumping out oxygen has a certain functional relationship withthe concentration of NOx in the measurement gas (referred to assensitivity characteristics hereinafter).

The sensitivity characteristics are previously specified using a pluraltypes of model gas whose NOx concentrations are already known in advanceof the actually use of the gas sensor 100, and data thereof is stored inthe controller 110. In the actual use of the gas sensor 100, signalsindicating a value of the NOx current Ip2 flowing in accordance with theNOx concentration in the measurement gas is provided to the controller110 from moment to moment, and the NOx concentration is continuouslycalculated based on the value and the specified sensitivitycharacteristics and output in the controller 110. According to the gassensor 100, the NOx concentration in the measurement gas can be obtainedalmost in real time.

<Relationship Between Main Pump Voltage and Resolution of NOx>

FIG. 2 is a drawing showing an influence of a magnitude of a main pumpvoltage Vp0 on a relationship between the oxygen concentration of themeasurement gas and an NOx current Ip2. Specifically, a measurement isperformed by two different gas sensors 100 on four types of model gashaving the same constant NO concentration of 500 ppm but having theoxygen concentration varied in four levels of 0%, 5%, 10%, and 18%,respectively (all having residual of N₂), and in FIG. 2, the NOx currentIp2 of the four types of model gas is plotted with respect to the oxygenconcentration of the model gas. The sensor element driving temperaturewas set to 830° C.

FIG. 2 shows graphs G1 and G2 corresponding to each of the two gassensors 100. The graph G1 shows a linear change of monotonic increase ina relationship between the NOx current Ip2 and the oxygen concentration,however, the graph G2 shows a tendency to monotonically increase in arange where the oxygen concentration is equal to or smaller than 10% butremains at the same level in a range where the oxygen concentration is10% to 18%.

Confirmed after an earnest review by the inventor of the presentinvention are findings that when the gas sensor 100 in which themagnitude of the main pump voltage Vp0 is kept equal to or smaller than650 mV during operation is applied therefor, a linear change ofmonotonic increase in which a determination coefficient (a square valueof a correlation function) R2 is equal to or larger than 0.975 such asthe graph G1 is obtained, however, when the gas sensor 100 in which themagnitude of the main pump voltage Vp0 is kept larger than 650 mV duringoperation is applied therefor, NOx current Ip2 has a tendency to remainat the same level such as the graph G2 in a range where the oxygenconcentration of the measurement gas is high. The determinationcoefficient R² of the latter case falls below 0.975.

The findings indicate that in the gas sensor 100 having theconfiguration that the magnitude of the main pump voltage Vp0 exceeds650 mV, NOx in the measurement gas is resolved before reaching themeasurement electrode 44 (in the first inner space 20, for example) whenthe oxygen concentration in the measurement gas is high.

Furthermore, it indicates that the sensor element 101 needs to have theconfiguration that the magnitude of the main pump voltage Vp0 is keptequal to or smaller than 650 mV in order to obtain the NOx concentrationaccurately even when the oxygen concentration in the measurement gas ishigh.

The graph G1 in FIG. 2 shows that the value of the NOx current Ip2 has atendency to depend on the oxygen concentration in the measurement gas.It suggests that the correction by the oxygen concentration is effectivein obtaining the NOx concentration based on the sensitivitycharacteristics in order to obtain the NOx concentration moreaccurately. It can be achieved by, for example, correcting the NOxcurrent Ip2 based on information indicating the oxygen concentration inthe measurement gas (for example, the pump current Ip0 or theelectromotive force V_(ref)).

<Suppression of Main Pump Voltage>

In consideration of the above points, defined in the gas sensor 100according to the present embodiment are requirements satisfied on adiffusion resistance from the gas inlet 10 to the inner pump electrode22, an electrical resistance of the main pump cell 21, an area of theinner pump electrode 22 constituting the main pump cell 21 and directlycontacting the measurement gas, and a shortest distance between theelectrodes in the main pump cell 21 (a shortest distance from the innerpump electrode 22 to the external pump electrode 23) from a viewpoint ofsuppressing the resolution of NOx in the first inner space 20 morereliably in the case where the oxygen concentration of the measurementgas is high. Since a contribution from the conduction part regarding theresolution of NOx may be negligible, thus the term “inner pump electrode22” indicates a part except for the conduction part in the followingdescription.

In the gas sensor 100 according to the present embodiment, by satisfyingrequirements (a) to (d) described below, the resolution of NOx in thefirst inner space 20 is suppressed even when the oxygen concentration ofthe measurement gas is high. Specifically, the gas sensor 100 has theconfiguration that the value of the main pump voltage Vp0 is kept equalto or smaller than 650 mV.

(a) Diffusion resistance from the gas inlet 10 to the inner pumpelectrode 22: equal to or larger than 200 cm⁻¹ and equal to or smallerthan 1000 cm⁻¹;

(b) Electrical resistance of the main pump cell 21: equal to or smallerthan 150Ω;

(c) Area of the inner pump electrode 22: equal to or larger than 5 mm²and equal to or smaller than 20 mm²;

(d) Shortest distance between electrodes of the main pump cell 21: equalto or larger than 0.1 mm and equal to or smaller than 0.6 mm.

If the ceiling electrode part 22 a is provided, the shortest distancebetween the electrodes is equal to a thickness of the second solidelectrolyte layer 6.

The diffusion resistance from the gas inlet 10 to the inner pumpelectrode 22 equal to or larger than 200 cm⁻¹ and equal to or smallerthan 1000 cm⁻¹ is achieved by appropriately combining the diffusionresistance of the first diffusion limiting part 11 and the diffusionresistance of the second diffusion limiting part 13.

The condition where the diffusion resistance exceeds 1000 cm⁻¹ is notpreferable by reason that a detection ability of oxygen is lowered. Inthe meanwhile, the condition where the diffusion resistance is smallerthan 200 cm⁻¹ is not preferable by reason that NOx is easily resolvedwith increase in the value of the pump current Ip0 and increase in thevalue of the main pump voltage Vp0, thus the detection accuracy islowered.

The condition where the electrical resistance exceeds 150Ω is notpreferable by reason that NOx is easily resolved with increase in thevalue of the main pump voltage Vp0, thus the detection accuracy islowered.

The condition where the area of the inner pump electrode 22 exceeds 20mm² is not preferable by reason that NOx is easily resolved in the firstinner space 20.

In the meanwhile, the condition where the area of the inner pumpelectrode 22 is smaller than 5 mm² or the condition where the shortestdistance between the electrodes exceeds 0.6 mm are not preferable byreason that an impedance of the main pump cell 21 increases, and thevalue of the main pump voltage Vp0 increases and NOx is easily resolved.

The condition where the shortest distance between the electrodes issmaller than 0.1 mm is not preferable by reason that a thickness of thesolid electrolyte located between the electrodes is reduced, thus acrack easily occurs.

Even in a case the inner pump electrode 22 includes only one of theceiling electrode part 22 a and the bottom electrode part 22 b asdescribed above, the resolution of NOx in the first inner space 20 inwhich the oxygen concentration of the measurement gas is high issuppressed as long as the requirements (a) to (d) described above aresatisfied.

The gas sensor 100 configured to satisfy the requirements (a) to (d)described above is used under a condition where a temperature of theinner pump electrode 22 is equal to or higher than 700° C. and equal toor lower than 900° C. by setting a sensor element driving temperature toequal to or higher than 700° C. and equal to or lower than 900° C. Acondition where the temperature of the inner pump electrode 22 exceeds900° C. is not preferable by reason that NOx in the first inner space 20is easily resolved, thus an assumed straight-line rate is not ensured.The condition where the temperature is lower than 700° C. is notpreferable by reason that an impedance of the main pump cell 21increases and the detection accuracy of the pump current Ip0 is lowered,and the value of the main pump voltage Vp0 increases and NOx is easilyresolved.

<Manufacturing Process of Sensor Element>

Described next is a process of manufacturing the sensor element 101having the configuration and the feature described above. In the presentembodiment, the sensor element 101 is manufactured by forming alaminated body formed of green sheets containing an oxygen ionconductive solid electrolyte such as zirconia as a ceramic component,and then cutting and firing the laminated body.

Described hereinafter as an example is a case of manufacturing thesensor element 101 including the six layers illustrated in FIG. 1.Prepared in such a case are six green sheets corresponding to the firstsubstrate layer 1, the second substrate layer 2, the third substratelayer 3, the first solid electrolyte layer 4, the spacer layer 5, andthe second solid electrolyte layer 6. FIG. 3 is a drawing showing a flowof processing in manufacturing a sensor element 101.

In manufacturing the sensor element 101, firstly, a blank sheet (notshown) which is a green sheet on which no pattern is formed is prepared(Step S1). When the sensor element 101 including the six layers ismanufactured, six blank sheets are prepared to correspond to each layer.Particularly, for the second solid electrolyte layer 6, the green sheetwhose thickness comes to satisfy the requirement (d) and further therequirement (b) in the end is used.

The blank sheets have a plurality of sheet holes used for alignment inperforming a printing and laminating the sheets. The sheet hole ispreviously formed in the blank sheet through, for example, punchingprocessing using a punching device in a stage prior to the patternformation. Green sheets corresponding to layers including the innerspaces also include penetrating portions corresponding to the innerspaces, which are also provided by the similar punching processingpreviously. The penetrating portions are formed so that the requirement(a) is satisfied in the finally obtained sensor element 101. A thicknessof each blank sheet corresponding to each layer of the sensor element101 needs not be the same as each other.

After the blank sheet corresponding to each layer is prepared, thepattern printing and dry processing are performed on each blank sheet(Step S2). Formed specifically are patterns of various types ofelectrodes, a pattern of the fourth diffusion limiting part 45, patternsof the heater element 72 and the heater insulating layer 74, and apattern of an inner wiring not shown in the drawings. An application ora placement of a sublimation material for forming the first diffusionlimiting part 11, the second diffusion limiting part 13, and the thirddiffusion limiting part 30 is also performed at a timing of the patternprinting. The application or the placement is performed so that therequirement (a) is satisfied in the finally obtained sensor element 101.

The printing of each pattern is performed by applying a patternformation paste prepared in accordance with characteristics required foreach formation object on the blank sheet using a known screen printingtechnique. A known drying means can be used for drying processing afterthe printing.

Particularly, the paste for forming the inner pump electrode 22 isprepared so that the finally obtained inner pump electrode 22 satisfiesat least the requirements (b) to (c) and is applied.

After the pattern printing on each blank sheet is finished, processingof printing and drying an adhesive paste for laminating and attachingthe green sheet corresponding to each layer on and to one another isperformed (Step S3). A known screen printing technique can be used forprinting the adhesive paste, and a known drying means can be used fordrying processing after the printing.

Subsequently, the green sheets on which an adhesive agent has beenapplied are stacked in a predetermined order, and the stacked greensheets are crimped under a predetermined temperature condition andpressure condition to form one laminated body (Step S4). Specifically,the crimping is performed by stacking and holding the green sheets to belaminated on a predetermined laminating jig not shown while aligning thegreen sheets using the sheet holes, and then heating and pressurizingthe green sheets together with the laminating jig using a laminatingmachine such as a known oil hydraulic pressing machine. The pressure,the temperature, and the time for heating and pressurizing depend on thelaminating machine to be used, however, an appropriate condition may bedetermined to be able to achieve a favorable lamination.

When the laminated body is obtained as described above, subsequently,the laminated body is cut out at a plurality of locations to obtain anindividual unit (referred to as the element body) of the sensor element101 (Step S5).

The firing is performed on the element body at a firing temperature ofapproximately 1300° C. to 1500° C. (Step S6). The sensor element 101 isthereby manufactured. In other words, the sensor element 101 ismanufactured through integrally firing the solid electrolyte and theelectrode. The firing temperature is preferably set to 1200° C. to 1500°C. (for example, 1400° C.). The integrated firing is performed in theabove manner, thus each electrode has sufficient adhesion strength inthe sensor element 101.

The sensor element 101 obtained in such a manner is stored in apredetermined housing, and incorporated into a main body (not shown) ofthe gas sensor 100.

EXAMPLE

Twelve types of gas sensors 100 (No. 1 to No. 12) each having differentcombination of the a diffusion resistance from the gas inlet 10 to theinner pump electrode 22, an electrical resistance of the main pump cell21, an area of the inner pump electrode 22, and a shortest distance fromthe inner pump electrode 22 to the external pump electrode 23 (ashortest distance between electrodes) were manufactured, and ameasurement was performed by each gas sensor 100 on a measurement gashaving an oxygen concentration of 18% and an NO concentration of 500 ppm(having residual of N₂). Presence or absence of the resolution of NOx inthe first inner space 20 was determined based on the value of the mainpump voltage Vp0 at that time. The sensor element driving temperaturewas set to 830° C.

Specifically, the diffusion resistance, the electrical resistance, thearea, and the shortest distance between the electrodes are varied asdescribed hereinafter.

Diffusion resistance: eight levels of 100 cm⁻¹, 180 cm⁻¹, 200 cm⁻¹, 300cm⁻¹, 500 cm⁻¹, 600 cm⁻¹, 700 cm⁻¹, and 900 cm⁻¹;

Electrical resistance: eleven levels of 30Ω, 45Ω, 55Ω, 75Ω, 80Ω, 85Ω,90Ω, 100Ω, 140Ω, 150Ω, and 200Ω;

Area: eight levels of 4.0 mm², 5.0 mm², 6.0 mm², 7.3 mm², 7.5 mm², 9.0mm², 15.0 mm², and 20.0 mm²;

Shortest distance between electrodes: four levels of 0.2 mm, 0.3 mm, 0.4mm, and 0.6 mm.

Table 1 shows the condition and a determination result of the presenceor absence of the resolution of NOx on each gas sensor 100. All of thegas sensors 100 of No. 1 to No. 10 satisfy all of the requirements (a)to (d). In the meanwhile, the gas sensors 100 of No. 11 and No. 12 donot satisfy the requirement (a), and the gas sensor 100 of No. 12 doesnot further satisfy the requirement (b) and (c).

TABLE 1 Diffusion resistance Electrical Area of Shortest distance toinner resistance inner between pump of main pump electrodes of electrodepump cell electrode main pump cell Determi- NO. [cm⁻¹] [ohm] [mm²] [mm]nation 1 300 85 7.5 0.3 ◯ 2 500 55 7.5 0.2 ◯ 3 700 100 6.0 0.3 ◯ 4 600150 5.0 0.4 ◯ 5 900 80 7.5 0.3 ◯ 6 500 30 20.0 0.3 ◯ 7 300 45 15.0 0.6 ◯8 200 75 9.0 0.3 ◯ 9 500 100 7.5 0.4 ◯ 10 300 140 6.0 0.4 ◯ 11 100 907.3 0.3 X 12 180 200 4.0 0.4 X

The presence or absence of the resolution of NOx in the first innerspace 20 is determined as follows.

The gas sensor 100 that the main pump voltage thereof is specified equalto or smaller than 650 mV is determined that NOx is not resolved, anddenoted by “◯” (circle) in an item of “Determination” of thecorresponding gas sensor 100 in Table 1.

In the meanwhile, the gas sensor 100 which the main pump voltage thereofis specified larger than 650 mV is determined that NOx is resolved, anddenoted by “X” (cross mark) in an item of “Determination” of thecorresponding gas sensor 100 in Table 1.

As illustrated in Table 1, the gas sensors 100 of No. 1 to No. 10satisfying all of the requirements (a) to (d) were determined that NOxwas not resolved. In contrast, the gas sensor 100 of No. 11 which doesnot satisfy the requirement (a) and the gas sensor 100 of No. 12 whichdoes not satisfy the requirements (a) to (c) were determined that NOxwas resolved in the first inner space 20.

The result shows that the gas sensor 100 capable of accurately obtainingthe NOx concentration even when the oxygen concentration in themeasurement gas is high can be achieved by satisfying all of therequirements (a) to (d).

While the invention has been shown and described in detail, theforegoing description is in all aspects illustrative and notrestrictive. It is therefore understood that numerous modifications andvariations can be devised without departing from the scope of theinvention.

What is claimed is:
 1. A limiting current type gas sensor capable ofspecifying a concentration of NOx in a measurement gas, comprising: asensor element formed of an oxygen ion conductive solid electrolyte,wherein said sensor element includes: a gas inlet into which ameasurement gas is introduced from an outer space; a first inner spacecommunicated with said gas inlet under a predetermined diffusionresistance; a second inner space communicated with said first innerspace under a predetermined diffusion resistance; a main pump cell whichis an electrochemical pump cell constituted by an inner pump electrodeprovided to face said first inner space, an external pump electrodeprovided on a surface of said sensor element, and said solid electrolytelocated between said inner pump electrode and said external pumpelectrode; a measurement electrode provided to face said second innerspace and covered with a porous protection film providing apredetermined diffusion resistance, said measurement electrodefunctioning as a reduction catalyst for NOx; an atmospheric airintroduction layer into which atmospheric air is introduced from outsideof said sensor element as a reference gas; a reference electrode coveredwith said atmospheric air introduction layer; and a measurement pumpcell which is an electrochemical pump cell constituted by saidmeasurement electrode, said external pump electrode, and said solidelectrolyte located between said measurement electrode and said externalpump electrode; and a controller which receives signals corresponding toa magnitude of an NOx current flowing between said measurement electrodeand said external pump electrode in said measurement pump cell andspecifies a concentration of said NOx based on the magnitude of the NOxcurrent, wherein said main pump cell is configured and disposed to pumpout oxygen in said first inner space when a predetermined main pumpvoltage is applied between said inner pump electrode and said externalpump electrode, and pumps out oxygen in said measurement gas introducedinto said first inner space to lower oxygen partial pressure of saidmeasurement gas in said first inner space, said measurement pump cell isconfigured and disposed to pump out oxygen near said measurementelectrode when a predetermined pump voltage is applied between saidinner pump electrode and said external pump electrode, and pumps outoxygen generated by a reduction of NOx in said measurement gas reachingnear said measurement electrode in said measurement electrode, adiffusion resistance from said gas inlet to said inner pump electrode isequal to or larger than 200 cm⁻¹ and equal to or smaller than 1000 cm⁻¹,an electrical resistance of said main pump cell is equal to or smallerthan 150Ω, and a shortest distance from said inner pump electrode tosaid external pump electrode is equal to or larger than 0.1 mm and equalto or smaller than 0.6 mm, and said inner pump electrode is a cermetelectrode formed of an Au—Pt alloy and ZrO₂ having an area equal to orlarger than 5 mm² and equal to or smaller than 20 mm².
 2. The gas sensoraccording to claim 1, wherein said sensor element further includes: amain-pump-control sensor cell which is an electrochemical sensor cellconstituted by said inner pump electrode, said reference electrode, andsaid solid electrolyte located between said inner pump electrode andsaid reference electrode; an auxiliary pump cell which is anelectrochemical pump cell constituted by an auxiliary pump electrodeprovided to face said second inner space, said external pump electrode,and said solid electrolyte located between said auxiliary pump electrodeand said external pump electrode; an auxiliary-pump-control sensor cellwhich is an electrochemical sensor cell constituted by said auxiliarypump electrode, said reference electrode, and said solid electrolytelocated between said auxiliary pump electrode and said referenceelectrode; and a measurement-pump-control sensor cell which is anelectrochemical sensor cell constituted by said measurement electrode,said reference electrode, and said solid electrolyte located betweensaid measurement electrode and said reference electrode, wherein whensaid main pump cell pumps out oxygen in said measurement gas located insaid first inner space, said main pump voltage in accordance with anelectromotive force generated between said inner pump electrode and saidreference electrode in said main-pump-control sensor cell is appliedbetween said inner pump electrode and said external pump electrode, saidauxiliary pump cell is configured and disposed to pump out oxygen insaid measurement gas introduced into said second inner space when a pumpvoltage in accordance with an electromotive force generated between saidauxiliary pump electrode and said reference electrode in said auxiliarypump control sensor cell is applied between said auxiliary pumpelectrode and said external pump electrode, said measurement gas whoseoxygen partial pressure has been further lowered compared to oxygenpartial pressure in said first inner space through pumping out oxygenwith said auxiliary pump cell reaches said measurement electrode, andwhen said measurement pump cell pumps out oxygen generated in saidmeasurement electrode, a pump voltage in accordance with anelectromotive force generated between said measurement electrode andsaid reference electrode in said measurement pump control sensor cell isapplied between said measurement electrode and said external pumpelectrode.